I do a lot of pigeon shooting and people often ask why my dogs don’t pick up decoys when I send them for one that’s just been shot.
The reason is purely down to basic, early, training.
There’s no secret to it.
I always start a young dog’s training in the pigeon pattern by sending him out among the decoys with the command to “leave” whenever he tries to pick one up.
By continually sending the dog for a fresh bird and telling him to ‘leave’ the ones you don’t want picking he will eventually cotton on to what’s required.
In addition, an experienced dog will pick up your hand scent on the decoys you’ve put out yourself and will only show interest in those you’ve just shot and dropped in the pattern.
The only word of caution I would give is to guard against sending a young dog as soon as you shoot a bird, as this will often cause it to run-in without being told.